Basic principles and mechanisms of NSOM; Different scanning modes and systems of NSOM; General applications and advantages of NSOM.

Transcription

1 Lecture 16: Near-field Scanning Optical Microscopy (NSOM) Background of NSOM; Basic principles and mechanisms of NSOM; Basic components of a NSOM; Different scanning modes and systems of NSOM; General applications and advantages of NSOM.

2 Scanning probe microscopies STM AFM NSOM Atomic resolution; Molecular bonding. Atomic Interaction: Contact mode Non-contact mode Tapping (intermittent) mode Other Interactions: Electrostatic mode (scanning electrostatic potential microscope) Magnetic mode Chemical Force mode Direct correlation between nanostructures and optical properties tunable SCM (scanning confocal microscope) Single-molecule spectroscopy

3 What can NSOM do? STM measures electric current, and AFM measures forces, neither deals with light; Light is a crucial excitation source in both scientific research and mother nature systems (e.g. photosynthetic system). Scientific research fields: absorption, fluorescence, photoinduced electron transfer, light-emitting devices, photovoltaic cells.

4 Why NSOM? Light diffraction limit of conventional optical microscopy: λ/2, ~ 250 nm. Actually, in real cases, the optical resolution ~ λ, 500 nm; in contrast, NSOM offers higher resolution around 50 nm (or even < 30 nm), depending on tip aperture size. NSOM provides simultaneous measurements of the topography and optical properties (fluorescence) --- direct correlation between surface nanofeatures and optical/electronic properties. This is especially useful for the studying the inhomogeneous materials or surfaces, like nanoparticles, polymer blends, porous silicon, biological systems. A 500 nm wide nanobelt appears wider than real size under an optical microscope.

5 NSOM imaging: Direct correlation between nanostructure features and optical/electronic properties TiO 2 particles wrapped in PPV film Fluorescence quenching by TiO 2 particles NESMI Lab data

6 NSOM Operation System

7 Major components of NSOM Optical: Light source (lasers: CW and pulsed), Fibers, Mirrors, Lenses, Objectives (oil, large NA) Photon detectors (Photon-Multiplier, Avalanche Diode) Probe (tip) Mechanical: Translation stage, Piezo scanner Anti-vibration optical table Electrical: Scanning drivers for piezo scanner z distance control (feedback system) Amplifiers, Signal processors Software and Computer

8 What is Near-Field? requires a nanometer sized aperture (much smaller than the light wavelength). A specimen is scanned very close to the aperture. As long as the specimen remains within a distance less than the aperture diameter, an image with sub-wavelength resolution (aperture size) can be generated. There is a tradeoff between resolution and sensitivity (light intensity) --- aperture size cannot be too small.

9 What is Near-Field? ~ 50 nm ~ 1 nm Near-Field λ/2 ~ 300 nm Near-field: For high spatial resolution, the probe must be close to the sample

10 What is Near-Field? ~ 50 nm ~ 1 nm Near-Field λ/2 ~ 300 nm For high spatial resolution, the probe must be close to the sample

11 Feedback Mechanism 1: Shear Force with Tuning Fork

12 Feedback Mechanism 1: Shear Force with Tuning Fork

13 NSOM based on shear force mode Dan Higgins, Acc. Chem. Res. 2005, 38,

14 NSOM imaging of cleaned glass NESMI Lab data

15 NSOM imaging of cleaned glass NESMI Lab data

16 Feedback Mechanism 2: AFM force sensor Compared to force sensor: Poor spatial resolution due to the fiber cantilever (reflection and spring constant); Damage to sample due to tapping scanning. Nanonics Imaging Ltd.

17 Feedback Mechanism 2: AFM force sensor

18 Structure of a NSOM Tip?

19 Different Operation Modes Illumination by the tip is probably the easiest to operate and interpret, and gives the most signal. It requires a transparent sample, so is limited for application in many samples like silicons and bio-species. Reflection modes give less light, and are more dependent on the details of the probe tip, but allow one to study opaque samples. The illumination/collection mode provides a complement to the reflection modes, but the signal contains a large background.

20 Different Operation Modes

21 Brief History of NSOM Ideas started in mid-1980 s; D.W. Pohl, W. Denk, and M. Lanz, Appl. Phys. Lett. 44, (1984). A. Lewis, M. Isaacson, A. Harootunian, and A. Murray, Ultramicroscopy 13, 227 (1984); Technology developed in 1990 s; Eric Betzig, et al. Science, 262, (1993). Eric Betzig, et al. Nature, 2369, (1994). Prototype commercial available since 2000 s;

22 Quick Looking back : Home Build NSOMs

23 New versions of NSOM 4 companies over the world produce good NSOM systems. The picture shows the model of Veeco Aurora III (DI, Thermomicro). The Aurora III installed in Zang Lab is the newest version (launched in January 2003). This one is based on shear force feedback.

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26 Analytical Chemistry, 2003, vol.75, page A

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28 Fabrication of NSOM Tip: chemical etching

29 Fabrication of NSOM Tip: mechanical pulling

30 Major applications of NSOM

31 Extended NSOM system spectral and optical imaging

32 Typical examples of NSOM research Quantum size effect for semiconductor nanocrystals; Self-organized nanostructures: thin film dewetting, phase separation; Self-assembly or self-alignment: nanospheres, nanorods, nanowires; Heterogeneous biological systems: cells, proteins, enzymes, membranes; Real optoelectronic devices: solar cells, optical switches, LEDs; Imaging single-molecules (research of SMS was initiated by NSOM); High resolution studies of charge transfer in DNA and polymer chains. To be discussed in the coming lectures

33 Nanoparticles: a unique manifestation Semiconductor nanospheres represent one of the most attractive nanostructures, and have a wide variety of applications in optoelectronics, magnetics, and biological applications. The size of nanoparticles can be tuned between individual molecules and the bulk counterparts. The nanosphere remains the same crystalline structure as the bulk crystal, but it shows unique size dependent physical and chemical properties, so called quantum size effect. See next slide. Conventional spectroscopy measurements of nanoparticles require uniform size distribution of the particle system, which is normally hard to attain. However, NSOM measurement removes such a need by focusing on only one particle a time.

34 Quantum Size Effect of Semiconductor Materials

35 Quantum size effect: an extensively research topic 1. As particle decreases in size, the bandgap increases, approaching the energy difference between LUOM and HOMO for the individual molecules; 2. For fluorescent semiconductor materials, like CdS, the different bandgap leads to different emission wavelength; 3. By making different sizes of the particles, people can tune the emission color across the whole visible region.

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37 Organic Semiconductor Nanocrystals: Q-effect less papers on organic nanoparticles, while thousands on inorganic counterparts. Why? CdSe O N O O N O Non-fluorescent fluorescent fluorescent fluorescent Inorganic Nanoparticle Organic Nanoparticle

38 Emission shift of PTCDI molecules upon crystal formation O N O O N O Emission (a.u.) CH 3 OH 38:62 H 2 O:CH 3 OH Quantum size effect is expected for the transition state between the two. crystal Wavelength (nm) NESMI Lab data

39 Tuning Emission of PTCDI materials Free molecules crystals NESMI Lab data

40 Spatially resolving quantum wells GaAs/AlGaAs quantum well structure

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43 Typical NSOM Examples: J-Aggregates of dyes

44 Typical NSOM Examples: Muscle Tissues

45 Typical NSOM Examples: Single-Molecules Embedded in Polymer Films

46 Revealing double strands of DNA: fluorescence dye YOYO-1 only combines with the double-strand

47 NSOM imaging in water: an approach to living cells

48 NSOM imaging of living cells

49 Aperturelss NSOM: tip-sample interaction in the polarizable sphere approximation model. when excited by an external radiation beam (wave vector k, fieldintensity E 0 ), the tip (radius of curvature a), gets polarized. The tip-dipole field p induces an image dipole field p located inside the sample at a distance 2r from the tip centre. The backreflected light contains information about the effective polarizability of the tip-sample system.

50 Some limitations (disadvantages) of NSOM Practically zero working distance (for objective) and an extremely small depth of field (for tip). Extremely long scan times for high resolution images or large specimen areas. Very low transmissivity of apertures smaller than the incident light wavelength --- low intensity of incident light for excitation, a problem for weak fluorescent molecules. Only surface features can be imaged and studied. Fiber optic probes are somewhat problematic for imaging soft materials due to their high spring constants, especially in shear-force mode